1. Field of the Invention
The present invention relates to a symbol timing synchronization, and more particularly, to a method for symbol timing synchronization suitable for a receiver in an orthogonal frequency division multiplexing (OFDM) system and an apparatus applying the same.
2. Description of the Related Art
The receiver in the OFDM communication system is to detect the start position of each symbol in order to correctly recovery the information transmitted by the transmitter, such operation is referred to as the “symbol timing synchronization”. For some OFDM systems, such as the WLAN (Wireless Local Area Network) or the DAB (Digital Audio Broadcast) systems, the transmitter in such system transmits some preamble or training sequence, such that the receiver can use it for the symbol timing synchronization. However, for some other OFDM systems, such as the DVB-T system, the transmitter does not transmit such preamble or training sequence. These systems usually utilize the characteristic of the guard intervals to detect symbol timing.
FIG. 1 is a schematic diagram illustrating how the conventional OFDM system uses guard intervals to detect the symbol timing in the AWGN (Additive White Gaussian Noise) channel. Referring to FIG. 1, r[n] represents a sample sequence of the received OFDM signal through a analog-to-digital conversion, and r[n−N] represents a sequence obtained from delaying the sample sequence r[n] by N sampling points. Wherein, the sample sequence r[n] is composed of a plurality of symbols, e.g. the symbols 110 and 120, and the delayed sequence r[n−N] is composed of a plurality of symbols, e.g. the symbols 110′ and 120′. Each symbol is composed of a guard interval of a length Ng (or has Ng sample points) and a useful data of a length N (or has N sampling points). The guard interval is in front of the useful data and is copied from an end part of the useful data.
For example, the guard interval 111 of the symbol 110 is copied from the end part 112 of the useful data, and the symbol 110′ is obtained from delaying the symbol 110 by N sampling points. The data in the guard interval 111′ is the same as the data in the guard interval 111 (the only difference is the existence of a delay gap), and the guard interval 111 is copied from the end part 112. Accordingly, the data in the guard interval 111′ is the same as the data in the end part 112. Similarly, the data in the end part 112′ of the symbol 110′ is the same as the data in the end part 112 of the symbol 110 (the only difference is the existence of a delay gap).
Since the duplicate structures (e.g. the guard interval 111 copied from the end part 112) is disposed in each symbol, some correlation exists between the sequences r[n] and r[n−N]. For example, a correlation sequence j[n] can be calculated from multiplying a complex conjugate of one of the sequence r[n] and r[n−N] by the either one of the sequence r[n] and r[n−N], i.e. (r* [n]×r[n−N]) or (r[n]×r* [n−N]). It is found that there is a high correlation between the sequences r[n] and r[n−N] during the interval 113 in the correlation sequence j[n]. Finally, a moving average of the correlation sequence j[n] is calculated to generate a cross-correlation sequence c[n], and the position of a peak 114 (i.e. the peak position) is detected in order to obtain a correct start position of the next symbol (e.g. the symbol 120).
In addition, in U.S. Pat. No. 6,088,406, the peaks of the cross-correlation sequence are more distinct by accumulating the correlation sequences of the multiple symbols, such that the reliability of the detection of the peak position is improved. However, the reliability of the method for detecting the symbol timing mentioned above is insufficient in the time-dispersive channel. Especially for the time-dispersive channel with long echo delay, such as the single frequency network (SFN) channel that is commonly used in the broadcast system, its reliability is not satisfactory. This is because the peaks of the cross-correlation sequence in the SFN channel are not as distinct as in the AWGN channel.
FIG. 2 is a schematic diagram of applying the method for symbol timing synchronization of FIG. 1 in the SFN channel with two paths. Wherein, these two paths have the same gain, and the path difference between them is Ng. Accordingly, after the OFDM signal had passed through the SFN channel, the receiver obtains a sample sequence r1[n] from one path and obtains a sample sequence r2[n] from the other path. In other words, the sample sequence r[n] is composed of the sequences r1[n] and r2[n].
A symbol 210 in the sample sequence r1[n] is exemplified herein. The symbol 220 in the sample sequence r2[n] has the same data as the symbol 210 in the sample sequence r1[n]. After the correlation and the moving average of the sequences r1[n] and r2[n] is calculated respectively, the sequences c1[n] and c2[n] are generated respectively. Therefore, the cross-correlation sequence c[n] is regarded as a combination of the sequences c1[n] and c2[n]. Comparing to the cross-correlation sequence c[n] in the AWGN shown in FIG. 1, the cross-correlation sequence c[n] in the SFN channel shown in FIG. 2 has a peak region rather than a distinct peak. Ideally, all sampling points in the peak region have the same value, i.e. the maximum value. Unfortunately, the values of the sampling points in the peak region are fluctuated due to noise and interference signal, so it is more difficult to detect the correct symbol position 214.
To resolve the drawback of the above-mentioned symbol timing synchronization method such as the symbol position is not easily detected in the time-dispersive channel of the SFN channel, another method is disclosed in U.S. Pat. No. 6,421,401. In the proposed method, the correlation is calculated by two correlation calculators, thus a more distinct peak is provided. However, it is obvious that such method requires additional correlation calculators, which means more multipliers and memory devices are required, thus the configuration is more complicated. In addition, in the thesis of “Enhanced symbol synchronization method for OFDM system in SFN channels” proposed by A. Palin and J. Rinne in IEEE Globecom Conference, 1998, it discloses that the symbol timing can be evaluated by determining whether the amplitude of the sampling points in the cross-correlation sequence has increased to a level exceeding a predetermined threshold. However, in different types of time-dispersive channels, the amplitude ranges of the sampling points in the cross-correlation sequence are not the same. Thus, it is hard to choose an appropriate threshold that is suitable for all types of time-dispersive channels.